Method for modeling coding information of video signal for compressing/decompressing coding information

ABSTRACT

A method for context-modeling coding information of a video signal for compressing or decompressing the coding information is provided. An initial value of a function for probability coding of coding information of a video signal of an enhanced layer is determined based on coding information of a video signal of a base layer.

1. TECHNICAL FIELD

The present invention relates to a method for modeling codinginformation of a video signal in order to compress the codinginformation or decompress the compressed information.

2. BACKGROUND ART

Scalable Video Codec (SVC) encodes video into a sequence of pictureswith the highest image quality while ensuring that part of the encodedpicture sequence (specifically, a partial sequence of framesintermittently selected from the total sequence of frames) can bedecoded and used to represent the video with a low image quality.

Although it is possible to represent low image-quality video byreceiving and processing part of a sequence of pictures encodedaccording to a scalable scheme, there is still a problem in that theimage quality is significantly reduced if the bitrate is lowered. Onesolution to this problem is to provide an auxiliary picture sequence forlow bitrates, for example, a sequence of pictures that have a smallscreen size and/or a low frame rate.

The auxiliary picture sequence is referred to as a base layer, and themain frame sequence is referred to as an enhanced or enhancement layer.When the base layer is provided, inter-layer prediction is performed toincrease coding efficiency.

The SVC, which is a codec extended from Advanced Video Codec (AVC) (alsoreferred to as ‘H.264’), can use Context-Adaptive Binary ArithmeticCoding (CABAC) for data compression. The CABAC is an entropy codingscheme which has been suggested for use in compression of a video signalcoded in AVC.

The CABAC performs modeling for arithmetic coding of coding informationof a macroblock, which is a specific syntax element, simply usingcorresponding coding information of an adjacent macroblock in the samelayer. Appropriate modeling of coding information according to theprobabilities of values of the coding information increases thecompression rate of arithmetic coding.

However, the CABAC has not defined a standard for modeling codinginformation using information of a plurality of layers. Thus, when SVCproviding a plurality of layers is employed, the CABAC cannot providesuitable modeling for coding information having a correlation betweenthe plurality of layers.

3. DISCLOSURE OF INVENTION

The present invention has been made in view of the above problems, andit is an object of the present invention to provide a method formodeling coding information having a correlation between a plurality oflayers in order to increase data compression rate.

In accordance with the present invention, the above and other objectscan be accomplished by the provision of a modeling method, wherein aninitial value of a function for probability coding of coding informationof an image block in a first layer is determined based on codinginformation of a second layer different from the first layer.

In an embodiment of the present invention, flags, which can be used fordetermining the initial value of the function for probability coding ofthe coding information, include a flag “base_mode_flag” indicatingwhether or not image data coded in an intra mode or motion vectorinformation of a block in the second layer corresponding to the imageblock is to be used for the image block, a flag“base_mode_refinement_flag” indicating whether or not refinement isnecessary for the image block to use motion vector information of ablock in the second layer corresponding to the image block, a flag“residual_prediction_flag” indicating whether or not residual data ofthe image block has been coded using data predicted from residual dataof a block in the second layer corresponding to the image block, a flag“intra_base_flag” indicating whether or not image data of the imageblock has been coded into difference data based on image data of anintra mode block in the second layer corresponding to the image block, aflag “mvd_ref_lX” indicating a refinement value required to obtain amotion vector of the image block using motion vector information of ablock in the second layer corresponding to the image block, and a flag“motion_prediction_flag_lX” indicating whether or not a motion vector ofa block in the second layer corresponding to the image block is to beused for a predictive motion vector of the image block.

In another embodiment of the present invention, coding information,corresponding to the coding information of the image block, of a blockin the second layer corresponding to the image block is used as thecoding information of the second layer.

In another embodiment of the present invention, information indicatingwhether a block in the second layer corresponding to the image block hasbeen coded in an inter mode or in an intra mode is used as the codinginformation of the second layer.

In another embodiment of the present invention, the initial value isdetermined based on whether or not partition information of a block inthe second layer corresponding to the image block is identical topartition information of the image block.

In another embodiment of the present invention, the initial value isdetermined based on a quantization parameter value of a block in thesecond layer corresponding to the image block or based on the differencebetween the quantization parameter value and a quantization parametervalue of the image block.

In another embodiment of the present invention, the initial value isdetermined based on the difference between a motion vector of the imageblock and a motion vector of a block in the second layer correspondingto the image block.

In another embodiment of the present invention, the initial value isdetermined based on whether or not a value indicating a referencepicture of the image block is identical to a value indicating areference picture of a block in the second layer corresponding to theimage block.

In another embodiment of the present invention, the initial value isdetermined based on whether or not spatial resolution of the image blockis identical to spatial resolution of a block in the second layercorresponding to the image block.

In another embodiment of the present invention, the initial value isdetermined based on block pattern information indicating whether or nota value other than 0 is present in a block in the second layercorresponding to the image block.

In another embodiment of the present invention, in order to determinethe initial value, two constants “m” and “n” are selected based on thecoding information of the second layer, an intermediate value isdetermined based on the selected constants “m” and “n” and informationassociated with coding of the second layer (for example, a quantizationparameter value of the second layer, the difference between quantizationparameter values of the first and second layers, or a spatial resolutionratio between the first and second layers), and an initial probabilityvalue and an MPS of the coding information are determined based onwhether or not the intermediate value is greater than a predeterminedvalue.

4. BRIEF DESCRIPTION OF DRAWINGS

The above and other objects, features and other advantages of thepresent invention will be more clearly understood from the followingdetailed description taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 is a block diagram of a CABAC executer of an enhanced layerencoder, which performs context modeling according to the presentinvention;

FIG. 2 illustrates an example of binarization of input codinginformation;

FIG. 3 illustrates a context modeling method for probability coding ofcoding information;

FIG. 4 illustrates a routine for determining an initial value forprobability coding; and

FIG. 5 is a block diagram of a CABAC executer of an enhanced layerdecoder, which performs context modeling according to the presentinvention.

5. MODES FOR CARRYING OUT THE INVENTION

The embodiments of the present invention will now be described in detailwith reference to the accompanying drawings.

FIG. 1 is a block diagram of a CABAC executer of the enhanced layeraccording to a preferred embodiment of the present invention. The CABACexecuter shown in FIG. 1 includes a binarizer 101, a context modeler102, and an arithmetic coder 110. The binarizer 101 binarizes inputnon-binary valued coding information according to a specified scheme.Specifically, the binarizer 101 converts a non-binary valued syntaxelement into a bin string as illustrated in FIG. 1. The context modeler102 models each bit of binary valued coding information based not onlyon coding information in an adjacent block in the same layer (in theenhanced layer in this example) but also on base layer codinginformation correlated with the binary valued coding information orinterlayer relationship information 10. The arithmetic coder 110performs arithmetic coding on an input bit based on a set model.

The arithmetic coder 110 includes a regular coding engine 103 forperforming arithmetic coding on bits of the coding information based onvariables (specifically, probability functions and initial values of theprobability functions) modeled by the context modeler 102, and a bypasscoding engine 104 for performing arithmetic coding on coding informationwhich does not benefit from modeling since bits 0 and 1 of the codinginformation have almost the same probability of occurrence.

The present invention, which involves modeling of input codinginformation, is not directly related to the arithmetic coding procedurebased on the modeled variables. Thus, a description of bit compression(entropy coding) associated with the arithmetic coder 110 is omittedherein since it is not necessary for understanding the presentinvention.

When the input coding information has a non-binary value, the CABACexecuter of FIG. 1 binarizes the value of the input coding informationthrough the binarizer 101. FIG. 2 illustrates an example of thebinarization. Coding information used in the example of FIG. 2 isassociated with macroblock types (mb_type). The macroblock types(Direct, Intra, P_(—)16×16, P_(—)16×8, P_(—)8×16, and P_(—)8×8) areassigned respective binary values (or bin strings) according to apredetermined scheme (or transform table). Other coding information isbinarized according to other schemes (or transform tables) specified forcorresponding elements in a similar manner to that of FIG. 2.

For bit compression, bits obtained through the above binarization areinput to the arithmetic coder 110 provided subsequent to the binarizer101. Bits of coding information, which have the same probability ofoccurrence of bit values 0 and 1, are directly input to the bypasscoding engine 104, whereas bits of coding information, which havedifferent probabilities of occurrence of bit values 0 and 1, are inputto the context modeler 102 so that the input bits are subjected to amodeling process.

The context modeler 102 performs modeling of each bit of input codinginformation in the enhanced layer based on bit values of correspondingcoding information of an adjacent macroblock and/or base layer codinginformation correlated with the coding information and a value ofcorresponding coding information received from a base layer encoder (notshown) or based on information 10 regarding the relationship between theenhanced layer and the base layer. Modeling is a process of selecting aprobability function and determining an initial value of the probabilityfunction. As illustrated in FIG. 3, an offset value k=1, k, or k+1 isdetermined according to coding information so that a probabilityfunction f_(k−1), f_(k), or f_(k+1) of the coding information isselected, and a value of an index variable “ctxIdxInc” is determinedfrom the offset value according to information correlated with thecoding information. The same probability function can be used for all ofcoding information. In other words, a single probability function can beused irrespective of coding information. As the value of the indexvariable “ctxIdxInc” is determined, initial values “valMPS” and“pStateIdx” for use with the probability function are determined. As theinitial value “pStateIdx” is determined, an initial probability of theLPS (or MPS) is determined as illustrated in FIG. 3. Accordingly, theregular coding engine 103 provided subsequent to the context modeler 102codes (or compresses) each bit of the input coding information using theselected probability function, starting from the determined initialvalues “valMPS” and “pStateIdx”. Here, the pStateIdx is a probabilitystate information and the valMPS is most probable value information forthe probability fuction.

We now suggest detailed examples of a method for determining the valueof the index variable “ctxIdxInc” when the context modeler 102 performsmodeling. A variety of examples suggested below are simply examples ofthe method for modeling specific coding information of the enhancedlayer based on coding information correlated with the value of thespecific coding information or based on the relationship between theenhanced and base layers. Thus, the present invention is not limited tothe examples suggested below, and any method, which is characterized bymodeling coding information of the enhanced layer based on codinginformation correlated with an element value of the coding informationor based on the relationship between the enhanced and base layers, fallswithin the scope of the present invention.

First, a description will be given of a variety of methods fordetermining an index variable “ctxIdxInc” of a flag “base_mode_flag”,which indicates whether or not coding information (for example, motionvector information or intra-mode image data) of a block in the baselayer corresponding to a macroblock is to be used for the macroblock.

ctxIdxInc=condTermFlagA+condTermFlagB+condTermFlagBase  1-1).

Here, “A” and “B” denote adjacent macroblocks located on the upper andleft sides of a current macroblock X. A flag “condTermFlagN” (N=A or B)has a value of “0” if a macroblock N is not available or if a flag“base_mode_flag” of the macroblock N is 0, otherwise it has a value of“1”. Similarly, a flag “condTermFlagBase” has a value of “0” if a blockin the base layer corresponding to the current macroblock X is notpresent or if a flag “base_mode_flag” of the corresponding block is 0,otherwise it has a value of “1”. That is, a value of correspondingcoding information of the base layer is also used as a basis fordetermining a value of the index variable “ctxIdxInc”. This indicatesthat an initial value for probability coding varies depending on thevalue of the corresponding coding information of the base layer.

ctxIdxInc=condTermFlagA′+condTermFlagB′+condTermFlagBase′  1-2).

Here, a flag “conTermFlag′” of a block is assigned a value of “0” or “1”depending on whether the block is in an inter mode or in an intra mode.For example, a flag “condtermFlagBase'” has a value of “0” (or “1”) if ablock in the base layer corresponding to the current macroblock X is inan inter mode, and has a value of “1” (or “0”) if the correspondingblock is in an intra mode.

In this method, the (inter or intra mode) modes of the two adjacentblocks A and B and the corresponding block in the base layer are used asa basis for determining the initial value of the probability functionfor coding the bit of the flag “base_mode_flag”.

Alternatively, only the flag “condtermFlagBase'” (i.e., only the mode ofthe corresponding block in the base layer) may be used as a basis fordetermining the value of the index variable “ctxIdxInc” so that theinitial value varies depending only on the value of the flag“condtermFlagBase′”.

ctxIdxInc=(BaseBlkSize==EnhanceBlkSize)?1:0+condTermFlagA+condTermFlagB  1-3).

In this method, a value indicating whether or not a partition of a blockin the base layer is identical to that of a block in the enhanced layer(for example, a value of “1” when identical and a value of “0” when notidentical) or a value indicating whether or not the size of a block inthe base layer is equal to that of a corresponding block in the enhancedlayer (for example, a value of “1” when equal and a value of “0” whennot equal) is used as a basis for determining the initial value of theprobability function.

ctxIdxInc=condTermFlagA″+condTermFlagB″+condTermFlagBase″  1-4).

Here, a flag “condTermFlag″” of a block is assigned a value of “1” if aquantization parameter of the block has a value equal to or greater thana predetermined threshold, otherwise it is assigned a value of “0”. Inthis method, quantization parameters of the two adjacent blocks A and Band a quantization parameter of the corresponding block in the baselayer are used as a basis for determining the initial value of theprobability function for coding the bit of the flag “base_mode_flag”.

The flag “condTermFlag″” of a block can also be assigned a value of “1”or “0” depending on the difference between the value of a quantizationparameter of the block and the value of a different quantizationparameter, rather than depending on the quantization parameter of theblock, so that the value of the index “ctxIdxInc” is determined based onthe value of the flag “condTermFlag″”. For example, a flag“condTermFlagN″” of a block N is assigned a value of “1” if thedifference between the value of a quantization parameter of the block Nand the value of a quantization parameter of a block in the base layercorresponding, to the block N is equal to or greater than apredetermined threshold, otherwise the flag “condTermFlagN″” is assigneda value of “0”. In this example, “condTermFlagBase″” denotes a flagindicating whether the difference between the value of a quantizationparameter of the current block X and the value of a quantizationparameter of a block in the base layer corresponding to the currentblock X is greater or less than the predetermined threshold.

Alternatively, only the flag “condTermFlagBase″” (i.e., only the valueof the quantization parameter of the block in the base layercorresponding to the current block X (or only the difference between thevalue of the quantization parameter of the current block X and the valueof the quantization parameter of the corresponding block)) may be usedas a basis for determining the value of the index variable “ctxIdxInc”so that the initial value varies depending only on the value of the flag“condTermFlagBase″”.

ctxIdxInc=0 (if C≧threshold 1), 1 (if threshold 1>C≧threshold 2), 2 (ifC<threshold 2)  1-5).

Here, “C” denotes a motion vector of the corresponding block in the baselayer or the difference between the motion vector of the correspondingblock and a motion vector of one of the adjacent macroblocks or anaverage motion vector of the adjacent macroblocks.

That is, the motion vector of the base layer is used as a basis fordetermining the initial value of the probability function.

ctxIdxInc=(refIdx_(Enhance) L1==refIdx_(Base) L1)? 1:0+(refIdx_(Enhance)L0==refIdx_(Base) L0)? 1:0  1-6)

In this method, a value indicating whether or not indices of referencepicture refIdxL0 and refIdxL1 in picture groups L0 and L1 of amacroblock having coding information, which is to be currently coded,are equal to indices of reference pictures in picture groups L0 and L1of a corresponding block in the base layer (for example, a value of “2”when both the reference picture indices refIdxL0 and refIdxL1 in thepicture groups L0 and L1 are equal to those of the base layer, a valueof “1” when one of the reference picture indices refIdxL0 and refIdxL1is equal to a corresponding one of the base layer, and a value of “0”when none of the reference picture indices refIdxL0 and refIdxL1 isequal to those of the base layer) is used as a basis for determining theinitial value of the probability function, so that the initial valuevaries depending on the value indicating whether or not the referencepicture indices in the enhanced layer are equal to those in the baselayer.

A combination of the above methods (1-1 to 1-6), rather than one of theabove methods, may be used to determine the initial value of theprobability function for entropy coding of the flag “base_mode_flag”.

Next, a description will be given of a variety of methods fordetermining an index variable “ctxIdxInc” of a flag“base_mode_refinement_flag” indicating whether or not refinement isnecessary for a macroblock to use motion vector information of a blockin the base layer corresponding to the macroblock.

Since the flag “base_mode_refinement_flag” is not used when acorresponding macroblock in the base layer has been coded in an intramode, a method assuming the intra mode coding, for example, a methodsimilar to the above method 1-2) are not used for modeling of the bit ofthe flag “base_mode_refinement_flag”.

ctxIdxInc=condTermFlagA+condTermFlagB+condTermFlagBase  2-1).

A flag “condTermFlagN” (N=A or B) has a value of “0” if a macroblock Nis not available or if a flag “base_mode_refinement_flag” of themacroblock N is 0, otherwise it has a value of “1”. Similarly, a flag“condTermFlagBase” has a value of “0” if a block in the base layercorresponding to the current macroblock is not present or if a flag“base_mode_refinement_flag” of the corresponding block is 0, otherwiseit has a value of “1”. That is, a value of corresponding codinginformation of the base layer is used as a basis for determining a valueof the index variable “ctxIdxInc”.

ctxIdxInc=(BaseBlkSize==EnhanceBlkSize)?1:0+condTermFlagA+condTermFlagB  2-2).

This method is similar to the above method 1-3).

ctxIdxInc=condTermFlagA″ +condTermFlagB″ +condTermFlagBase″ orctxIdxInc=condTermFlagBase″  2-3).

This method is similar to the above method 1-4).

ctxIdxInc=(SpatialRes_(Enhance)==SpatialRes_(Base))? 1:0  2-4).

In this method, a value indicating whether or not spatial resolution ofa picture in the base layer is equal to that of a picture in theenhanced layer (for example, a value of “1” when equal and a value of“0” when not equal) is used as a basis for determining the initial valueof the probability function.

A combination of the above methods (2-1 to 2-4), rather than one of theabove methods, may be used to determine the initial value of theprobability function for probability coding of the flag“base_mode_refinement_flag”.

Next, a description will be given of a variety of methods fordetermining an index variable “ctxIdxInc” of a flag“residual_prediction_flag” indicating whether or not residual data of amacroblock has been coded using data predicted from residual data of ablock in the base layer corresponding to the macroblock.

ctxIdxInc=condTermFlagA+condTermFlagB+condTermFlagBase  3-1).

A flag “condTermFlagN” (N=A or B) has a value of “0” if a macroblock Nis not available or if a flag “residual_prediction_flag” of themacroblock N is 0, otherwise it has a value of “1”. Similarly, a flag“condTermFlagBase” has a value of “0” if a block in the base layercorresponding to the current macroblock is not present or if a flag“residualprediction_flag” of the corresponding block is 0, otherwise ithas a value of “1”. That is, a value of corresponding coding informationof the base layer is used as a basis for determining a value of theindex variable “ctxIdxInc”.

ctxIdxInc=(BaseBlkSize==EnhanceBlkSize)?1:0+condTermFlagA+condTermFlagB  3-2).

This method is similar to the above method 1-3).

ctxIdxInc=condTermFlagA″+condTermFlagB″+condTermFlagBase″ orctxIdxInc=condTermFlagBase″  3-3).

This method is similar to the above method 1-4).

ctxIdxInc=(refIdx_(Enhance) L1==refIdx_(Base) L1)? 1:0+(refIdx_(Enhance)L0==refIdx_(Base) L0)? 1:0  3-4).

This method is similar to the above method 1-6).

ctxIdxInc=(if C≧threshold 1), 1 (if threshold 1>C≧threshold 2), 2 (ifC<threshold 2)  3-5).

This method is similar to the above method 1-5).

ctxIdxInc=(SpatialRes_(Enhance)==SpatialRes_(Base))? 1:0  3-6).

This method is similar to the above method 2-4).

ctxIdxInc=CBP_(Base)? 1:0  3-7).

In this method, the initial value of the probability function for codingthe flag “residual_prediction_flag” is determined from a value of aCoded Block Pattern (CBP) of a corresponding block in the base layer.Here, a CBP of a luminance block or a chrominance block of the baselayer can be used as the CBP. The CBP of a block has a value other than“0” if any non-zero value is present in the block, otherwise it has avalue of “0”. In this method, the initial value of the probabilityfunction for coding the flag “residual_prediction_flag” is setdifferently depending on a value indicating whether or not a value otherthan “0” is present in the corresponding block in the base layer, whichis “1” when any non-zero is present and “0” when a value other than “0”is not present.

This case is explained in more detail. In the event that resolutionratio between current layer and base layer is 2:1, the size of a blockon the base layer corresponding to a current 16×16 macroblock is 4×4.Therefore, a bit value associated with the 4×4 block in the CBP of amacroblock including the 4×4 block on the base layer is set to the indexvariable “ctxIdxInc” to determine initial value of a probabilityfunction because the bit value in the CBP indicates whether the 4×4block has any non-zero value.

Further, if a macroblock ‘BL1_MB’ on the base layer including the 4×4block is coded in residual prediction manner based on a correspondingblock ‘BL2_B’ on a lower layer (the second base layer), that is, if“residual_prediction_flag” of the macroblock ‘BL1_MB’ on the base layeris set to “1”, value of the index variable “ctxIdxInc” is determinedaccording to an associated CBP bit of the corresponding block ‘BL2_B’ onthe second base layer.

In the method of using the CBP for determining the initial value, thevalues of corresponding coding information “residual_prediction_flag” ofadjacent blocks A and B, in addition to the above condition (CBP_(Base)?1:0), can be used as conditions for determining the index variable“ctxIdxInc”. In this case, the index variable “ctxIdxInc” can bedetermined as follows:

ctxIdxInc=CBPBase? 1:0+condTermFlagA+condTermFlagB

Alternatively, the index variable “ctxIdxInc” may be determined based onthe CBP values of the two adjacent blocks A and B as follows:

ctxIdxInc=CBP_(A)? 1:0+CBP_(B)? 1:0

A combination of the above methods (3-1 to 3-7), rather than one of theabove methods, may be used to determine the initial value of theprobability function for probability coding of the flag“residual_prediction_flag”.

Modeling (for example, initial value setting) of coding informationother than the coding information described above can also be performedin different manners, according to coding information of the base layeror according to an interlayer relationship.

For example, modeling for probability coding of a flag “intra_base_flag”indicating whether or not image data of an enhanced layer block has beencoded into difference data based on image data of an intra mode block inthe base layer corresponding to the enhanced layer block can also beperformed in different manners, using an interlayer relationship(specifically, using corresponding coding information in the base layer)according to a similar method to that of 1-1), using an interlayerrelationship of spatial resolution according to a similar method to thatof 2-4), or using a quantization parameter representing the imagequality level of the base layer according to a similar method to that of1-4).

In addition, modeling for probability coding of information“mvd_ref_lX,X=0,1” indicating a refinement value required for amacroblock to use motion vector information of a block in the base layercorresponding to the macroblock can also be performed in differentmanners, using an interlayer relationship (specifically, usingcorresponding coding information in the base layer) according to asimilar method to that of 1-1) or using an interlayer relationship ofspatial resolution according to a similar method to that of 2-4).

Further, modeling for probability coding of a flag“motion_prediction_flag_lX,X=0,1” indicating whether or not a motionvector of a block of the base layer corresponding to a macroblock is tobe used for a predictive motion vector of the macroblock can also beperformed in different manners, using an interlayer relationship(specifically, using corresponding coding information in the base layer)according to a similar method to that of 1-1), using an interlayerrelationship of spatial resolution according to a similar method to thatof 2-4), or using a block size relationship according to a similarmethod to that of 1-3).

The variety of modeling methods described above can also be applied toany other coding information whose value may be affected by theinterlayer relationship.

Although the above description has been given as if the initial values“valMPS” and “pStateIdx” are determined directly from the index variable“ctxIdxInc”, the two initial values are determined from values “m” and“n” which are determined from the index variable “ctxIdxInc” asillustrated in FIG. 4.

An intermediate value “preCtxState” in the initial value determinationroutine of FIG. 4 is determined by a function “Clip3( )”. ThepreCtxState determination function “Clip3( )” has a luminancequantization parameter “SliceQPY” as an argument (varX), in addition tothe values “m” and “n”. This argument (varX) is associated with amacroblock having coding information which is to be currently coded. Theargument (varX), which has an influence on the initial valuedetermination in addition to the values “m” and “n”, has no valueassociated with the interlayer relationship.

Accordingly, if the interlayer relationship is reflected in the argument(varX) so that an initial value based on the interlayer relationship isobtained, the initial value is likely to have a value which is much moreadvantageous in terms of probability coding. Thus, according to thepresent invention, the interlayer relationship is reflected in theargument (varX).

Methods for reflecting the interlayer relationship in the argument(varX) are to use a luminance quantization parameter “BaseSliceQPY” ofthe base layer as the argument (varX), to use the difference betweenquantization parameters of the enhanced and base layers as the argument(varX), or to use the spatial resolution ratio between the base layerand the current layer as the argument (varX).

When the difference between the quantization parameters of the enhancedand base layers is used as the argument (varX), the preCtxStatedetermination function 401 of FIG. 4 can be defined as follows accordingto the present invention:

preCtxState=Clip3(1,126,((m*(SliceQPYBaseSliceQPY))>>4)+n)

Although the above method has been described for probability coding inthe encoder, the same method is applied to a CABAC decoder of FIG. 5 fordecompressing compressed data, and a description of context modeling inthe decoder is omitted herein.

In a similar manner to the method whereby the context modeler 102 in theCABAC coder of FIG. 1 models target coding information, a contextmodeler 202 in the CABAC decoder of FIG. 5 models target codinginformation based on base layer coding information and interlayerrelationship information 20 and passes a corresponding initial value toa regular decoding engine 203 provided subsequent to the context modeler202. The regular decoding engine 203 converts bits of input codinginformation into a decompressed bit string, starting from the same valueas the initial value employed in the regular coding engine 103 in theencoder.

The decoder, which includes the context modeler for modeling codinginformation according to the method described above, can be incorporatedinto a mobile communication terminal, a media player, or the like.

As is apparent from the above description, a method for modeling codinginformation of a video signal for compressing and decompressing thecoding information according to the present invention performs contextmodeling of each bit of the coding information using an interlayercorrelation, so that an initial value of a function for probabilitycoding of the coding information, which is advantageous in terms ofprobability coding, (i.e., an initial value which further reduces theinitial probability of the LPS), is determined, thereby significantlyincreasing the data compression rate of probability coding.

Although this invention has been described with reference to thepreferred embodiments, it will be apparent to those skilled in the artthat various improvements, modifications, replacements, and additionscan be made in the invention without departing from the scope and spiritof the invention. Thus, it is intended that the invention cover theimprovements, modifications, replacements, and additions of theinvention, provided they come within the scope of the appended claimsand their equivalents.

1-26. (canceled)
 27. A method for modeling coding information,comprising: receiving a video signal including a bitstream of anenhanced layer and a bitstream of a base layer; extracting codinginformation of a neighbor block from the bitstream of the enhancedlayer, the coding information indicating whether an image block in theenhanced layer uses prediction information of a corresponding block inthe base layer, the corresponding block indicating a block referred bythe image block; deriving flag information based on the codinginformation of the neighbor block, the flag information indicating avalue determined based on the availability of the neighbor block and thecoding information of the neighbor block; deriving a first indexvariable using the flag information of the neighbor block; determiningan initial value of a function for probability coding based on the firstindex variable; and modeling the coding information of the image blockusing the initial value.
 28. The method of claim 1, wherein the neighborblock includes at least one of an upper side block of the image blockand a left side block of the image block.
 29. The method of claim 2,wherein the flag information has a value 0, when the upper side block orthe left side block is available and the coding information of the upperside block or the left side block indicates that the image block usesthe coding information of the corresponding block.
 30. The method ofclaim 2, wherein the flag information has a value 1, when the upper sideblock and the left side block is not available, or the codinginformation of the upper side block and the left side block indicatesthat the image block does not use the coding information of thecorresponding block.
 31. The method of claim 2, wherein the first indexvariable is derived by adding the flag information of the upper sideblock to the flag information of the left side block.
 32. The method ofclaim 1, wherein the determining step further comprises deriving asecond index variable using the context index offset for the image blockand the first index variable, wherein the initial value is determinedbased on the second index variable.
 33. The method of claim 6, whereinthe initial value of the function for probability coding is determinedusing table information, the table information being determined based onthe second index variable.
 34. The method of claim 1, wherein an imagequality or a spatial resolution of the enhanced layer is higher thanthat of the base layer.
 35. An apparatus for modeling codinginformation, comprising: a context modeler receiving a video signalincluding a bitstream of an enhanced layer and a bitstream of a baselayer, extracting coding information of a neighbor block from thebitstream of the enhanced layer, deriving flag information based on thecoding information of the neighbor block, deriving a first indexvariable using the flag information of the neighbor block, determiningan initial value of a function for probability coding based on the firstindex variable, and modeling the coding information of the image blockusing the intermediate value, wherein the coding information indicateswhether an image block in the enhanced layer uses prediction informationof a corresponding block in the base layer, the corresponding blockindicating a block referred by the image block, wherein the flaginformation indicates a value determined based on the availability ofthe neighbor block and the coding information of the neighbor block. 36.The apparatus of claim 9, wherein the neighbor block includes at leastone of an upper side block of the image block and a left side block ofthe image block.
 37. The apparatus of claim 10, wherein the flaginformation has a value 0, when the upper side block or the left sideblock is available and the coding information of the upper side block orthe left side block indicates that the image block uses the codinginformation of the corresponding block.
 38. The apparatus of claim 10,wherein the flag information has a value 1, when the upper side blockand the left side block is not available, or the coding information ofthe upper side block and the left side block indicates that the imageblock does not use the coding information of the corresponding block.39. The apparatus of claim 10, wherein the first index variable isderived by adding the flag information of the upper side block to theflag information of the left side block.